Method of protecting zinc-copper alloys against corrosion and product thereof



United States Patent METHOD OF PROTECTING ZINC-COPPER ALLOYS AGAINSTCORROSION AND PRODUCT THEREOF Allan E. Chester, Highland Park, and BrunoR. Jeremias, Chicago, Ill., and Joseph T. Irwin, Lakewood, Ohio,assignors to Poor & Company, Chicago, 111., a corporation of Delaware NoDrawing. Application April 15, 1953, Serial No. 349,105

6 Claims. (Cl. 148-62.)

This invention relates to corrosion protected articles and to a methodof protecting zinc-copper alloys, more particularly white alloys of zincand copper against corrosion.

White alloys of zinc and copper have a tendency to turn white or grayishwhite with age, a property sometimes referred to as white corrosion,which is apparently due to the formation of oxides and carbonates ofzinc.

Zinc surfaced articles have been protected against white corrosion bytreating them with various types of chromic acid solutions usuallyfollowed by a secondary treatment with an alkali to remove some of theby-products formed by the primary treatment. In our copendingapplication Serial No. 349,068, filed of even date herewith, we havedescribed a method of protecting chromium plated zinc-copper alloysurfaced articles by treatment with an acidified aqueous solution ofchromic acid. In order to obtain the desired corrosion protection it isnecessary to add enough acid to produce a relatively low pH and most ofthese acidified chromic acid solutions when applied to the zinc-copperalloy coating which has not been over-plated with chromium causediscoloration of the surface which is especially undesirable when abright finish of high specular reflectivity is required.

One of the objects of the present invention is to provide a method forprotecting against corrosion alloys of zinc and copper regardless ofwhether such. alloy have been over-plated with chromium.

and improved articles containing a surface coating of an alloy of zincand copper sealed with a sealing composition which affords goodprotection against corrosion and does not discolor the zinc-copper alloysurface.

Another object of the invention is to provide new and improved corrosioninhibiting compositions for protecting against corrosition alloys ofzinc and copper. Other objects will appear hereinafter.

In accordance with the invention it has been found that zinc-coppersurfaced articles can be protected against corrosiion by treating themwith a soluion of chromic acid (CrOz) and nitric acid, preferablycontaining an acid stable wetting agent.

The chromic acid-nitric acid solution is preferably prepared in the formof a concentrate and then diluted with water before being applied to theZinc-copper alloy surface of the article. The pH of the compositionwhich is applied to the zinc-copper alloy surfaced article is preferablywithin the range of 0.75 to 1.75.

The corrosion protection treatment may be applied by spraying, dippingor in any other suitable manner, but it is preferable to immerse thearticle to be treated containing a Zinc-copper alloy surface in asolution of the chromic acid and nitric acid composition for a shortperiod of time. Good results can usually be obtained by immersing thezinc-copper surfaced article in such a solution for a period of 1 minutebut the time period can be varied depending upon the concentration ofthe solution Patented June 18, 1957 EXAMPLE A concentrate is prepared bymixing together the 7 following ingredients in water:

Ounces per gallon CrOs 30 NfizCraOmZHzO 15 68% HNO3 2.1.5 40% aqueoussolution of octyl alcohol sodium sulfate .85

This concentrate is diluted with water to produce a sealing compositioncontaining 2% to 6% by volume of the foregoing concentrate (2-6 cc. ofconcentrate per cc. of aqueous solution) and articles having a whitezinc-copper alloy surface or a white ZlIlC'1COPPBl' alloy surfaceover-plated with chromium are protected against corrosition by immersingthem in this composition. The optimum results are obtained with a 4% byvolume solution of the concentrate at a temperature of 20 to 25 C. foran immersion period of about 1 minute.

In this composition a 2% solution by volume of the concentrate has a pHof about-1.3 and a 4% by volume solution of the concentrate has a pH ofabout 1.0.

In the foregoing example the octyl alcohol sodium sulfate is a wettingagent and can be omitted from the composition. lts primary purpose is toreduce the surface tension between the active eifecive ingredients ofthe composition and the zinc-copper alloy surface article. The sodiumdichromate in the composition is another source of chromic acid which isformed by the reaction between the dichromate and nitric acid producingsodium nitrate as an ingredient in the composition.

The acid content should be held within relatively narrow limits asdefined by the pH in order to obtain the desired results. Especiallysatisfactory results are obtained in the practice of the invention withwhite zinccopper alloy surfaced articles having a copper content in thealloy of 10% to 20%, the remainder being zinc.

The copper content can be as low as 5% and is usually not higher than30% by weight of the alloy. The corrosion inhibiting compositiondescribed in the foregoing example is primarily applicable to thetreatment of predominantly zinc alloys of zinc and copper which have asilvery or whitish appearance and can also be applied to surfaces ofsuch alloys that have been over-plated with chromium.

After the corrosion inhibiting treatment has been applied the resultantzinc-copper alloy surfaced article is rinsed simply by washing withwater. No secondary treatment is required either by leaching with analkali or otherwise.

If a wetting and penetrating agent is used in the sealing composition itshould be one which is stable under the highly acid conditions and atthe same time not precipitated from the bath or destroyed by the highconcenration of chromic acid present in the sealing composition.Examples of suitable wetting and penetrating agents are octyl alcoholsodium sulfate, isopropyl naphthalene sodium sulfonate, the dioctylester of sodium sulfosuccinic acid, isobutyl naphthalene sodiumsulfonate, other alkylated aryl sulfonic acids, soya bean amine oxyalkylated with approximately 10 mols of ethylene oxide, containing anaverage of 20 carbon atoms in its alkyl chain and having an averagemolecular weight of 714,

and other oxyalkylated amines of long chain acids containing 8 to 60carbon atoms in which the oxyalkyl groups are derived from ethyleneoxide, 1,2-propylene oxide or mixtures thereof. The preferred wettingagents of this last mentioned type contain at least 4 mols of alkyleneoxide per mol of primary fatty amine, preferably around 10 to 20 mols ofalkylene oxide per mol of primary fatty amine, and are soluble andstable in acids in the concen trations employed in the practice of theinvention.

The sealing composition employed in the practice of the inventionpreferably contains around 1 gram to 10 grams of chromium (Cr) aschromic acid ((3103) per liter of solution. Smaller amounts produce aless pronounced effect and larger amounts do not substantially add tothe improvement in the corrosion protection obtained and may producediscoloration.

The invention provides new and improved corrosion protected artices andmakes it possible to seal a coating of a zinc and copper alloy toenhance the corrosion resistance. it is especially valuable in providinga method for enhancing the corrosition resistance of white Zinccopperalloy surfaced articles which have a high specular reflectivity becausethe treatment does not discolor the surface. The same is true where sucharticles have been over-plated with chromium and especially where thecoating of chromium is non-uniform, as is often the case with contouredarticles because of the poor throwing power of chromium electroplatingbaths. An important advan tage of the treatment herein described is thatit is unnecessary to heat either the zinc-copper alloy surfaced articleor the sealing composition which is applied thereto. Zinc-copper alloysurfaced articles which have been treated in accordance with thepractice of the invention are highly resistant to white corrosion and,in the case of ferrous metal articles, to the formation of red rustcorrosion over a long period of time.

The expression zinc-copper alloy surfaced article is employed herein tocover any article having an exterior surface of an alloy of zinc andcopper regardless of whether the article itself is made of zinc, azinc-copper alloy, a ferrous metal, or some other metal capable of beingcoated with a zinc-copper alloy. The expression zinc-copper alloy platedarticle is employed herein to describe a zinc-copper alloy coatedarticle in which the zinc-copper alloy coating has been applied by anelectroplating process. The expression white alloy of zinc and copper isemployed herein to describe alloys of zinc and copper which have a whiteor silvery appearance as distinguished from alloys of zinc and copperwhich are yellow in appearance. The expression overplated with chromiumis employed herein to describe an article in which a chromium layer orcoating has been applied by electrodeposition over the zinc-copper alloysurfaced article.

The invention is hereby claimed as follows:

1. A process of improving corrosion resistance which comprises treatinga zinc-copper alloy surfaced article wherein the alloy contains 5% to30% by weight copper, the remainder being zinc, with an aqueous solutionconsisting essentially of chromic acid and nitric acid having a pHwithin the range of 0.75 to 1.75 and containing 1 to 10 grams ofchromium per liter of solution.

2. A process of improving corrosion resistance which comprises treatinga zinc-copper alloy surfaced article wherein the alloy contains 5% to30% by weight copper, the remainder being zinc, with an aqueous solutionconsisting essentially of chromic acid, sodium dichromate and nitricacid having a pH within the range of 0.75 to 1.75 and containing 1 to 10grams of chromium per liter of solution.

3. A process of improving corrosion resistance which comprises treatinga zinc-copper alloy surfaced article wherein the alloy contains 57% to30% by weight copper, the remainder being zinc, with an aqueous solutionconsisting essentially of chromic acid and nitric acid, containing 1 to10 grams of chromium per liter of solution, having a pH within theran-geof 0.75 to 1.75 and containing a wetting agent which is stable insaid acid solution.

4. A process of improving corrosion resistance which comprises treatinga zinc-copper alloy surfaced article wherein the alloy contains 5% to30% by Weight copper, the remainder being Zinc, with an aqueous solutionconsisting essentially of chromic acid and nitric acid, containing l to10 grams of chromium per liter of solution, having a pH Within the rangeof 0.75 to 1.75 for a period of 10 seconds to 2 minutes and rinsing theresultant prodact with water.

5. A zinc-copper alloy surfaced article wherein the alloy contains 5% to30% by weight copper, the remainder being zinc, inhibited againstcorrosion with a coating of an aqueous solution consisting essentiallyof chromic acid and nitric acid having a pH within the range of 0.75 to1.75 and containing 1 to 10 grams of chromium per liter of solution.

6. A white zinc-copper alloy surfaced article wherein the alloy contains10% to 20% by weight copper, the remainder being zinc, inhibited againstcorrosion with a sealing coating consisting essentially of an aqueoussolution of chromic acid and nitric acid having a pH within the range of0.75 to 1.75 and containing 1 to 10 grams of chromium per liter ofsolution.

References Cited in the file of this patent UNITED STATES PATENTSGermany May 24,

1. A PROCESS OF IMPROVING CORROSION RESISTANCE WHICH COMPRISING TREATINGA ZINC-COPPER ALLOY SURFACED ARTICLE WHEREIN THE ALLOY CONTAINS 5% TO30% BY WEIGHT COPPER, THE REMAINDER BEING ZINC, WITH AN AQUEOUS SOLUTIONCONSISTING ESSENTIALLY OF CHROMIC ACID AND NITRIC ACID HAVING A PHWITHIN THE RANGE OF 0.75 TO 1.75 AND CONTAINING 1 TO 10 GRAMS OFCHROMIUM PER LITER OF SOLUTION.